Developmental NMDA receptor signaling regulates cerebellar unipolar brush cell number and dampens excitability

Unipolar brush cells (UBCs) are a type of excitatory interneuron found in the cerebellum, characterized by a unique dendritic brush that amplifies and transmits incoming signals. These cells rely on both ionotropic and metabotropic glutamate receptors to process synaptic input. The development of UBCs involves complex regulation of these receptors, though the exact mechanisms and significance remain unclear.

NMDA-type glutamate receptors (NMDARs) play a crucial role in both development and plasticity in UBCs. Researchers analyzed the expression of NMDAR subunits during early postnatal development and found that tonic and synaptic NMDAR-mediated currents are present in these cells. However, the expression of these receptors decreases as the cells mature.

Research using whole-cell patch clamp recordings revealed that NMDARs contribute to both synaptic signaling and a tonic calcium flux that helps regulate excitability in developing UBCs.

Interestingly, genetic deletion or pharmacological blockade of NMDARs led to an unexpected increase in the number of UBCs in adulthood. This finding suggests that NMDARs might be involved in programmed cell death pathways. The paradoxical effects on excitability were calcium dependent and were counteracted by inhibiting calcium-activated potassium channels.

In summary, while NMDARs are not essential for the migration and dendritic development of UBCs, they play critical roles in synaptic signaling and providing a tonic calcium flux that dampens excitability in these cells. Their functions may also influence the transformation of vestibular signals, which are essential for smooth movements and balance.